In recent years, a range of electronic devices, including mobile phones and electronic cameras, are becoming more sophisticated and diversified. There are increasingly more electronic devices equipped with a light-transmissive touch panel attached to a front surface of a display element such as liquid crystal. Diversifying functions of an electronic device is switched over as the user operates the touch panel typically by finger while looking at indication of display element at the back via this touch panel. Therefore, the user demands touch panels that can be easily and reliably operated.
FIG. 3 is an exploded perspective view of conventional touch panel 500. Plural strip-like light-transmissive upper conductive layers 502 made of indium tin oxide are arranged on an upper surface of film-like light-transmissive upper substrate 501 in a front-back direction. Plural upper electrodes 503 made of copper foils are connected to front ends or back ends of upper conductive layers 502. An end of each upper electrode 503 is led out to a right peripheral end of upper substrate 501. Plural upper electrodes 503 extend in a left-right direction perpendicular to upper conductive layers 502.
Plural strip-like light-transmissive lower conductive layers 505 made of indium tin oxide are arranged on an upper surface of film-like light-transmissive lower substrate 504. Plural lower conductive layers 505 are arranged in the left-right direction perpendicular to upper conductive layers 502. Plural lower electrodes 506 made of copper foils are connected to right ends of lower conductive layers 505. An end of each of lower electrodes 506 is led out to a right peripheral end of lower substrate 504. Lower electrodes 506 extend in the left-right direction parallel to lower conductive layers 505.
Cover 507 is light transmissive and is made of insulating material, such as glass or polycarbonate. Upper substrate 501 is laid over the upper surface of lower substrate 504, and cover 507 is laid over the upper surface of upper substrate 501. Lower substrate 504, upper substrate 501, and cover 507 are adhered to each other by adhesive layer 508 made of, e.g. acryl or rubber, formed on the upper surfaces of upper substrate 501 and lower substrate 504, providing touch panel 500.
Touch panel 500 is disposed on a front surface of a display element, such as a liquid crystal device, and installed in an electronic device. The ends of upper electrodes 503 and lower electrode 506 led out to the right peripheral end are electrically connected to an electronic circuit of the device via a flexible wiring board and a connector.
While a voltage is applied from the electronic circuit sequentially to upper electrodes 503 and lower electrodes 506, a user operates the electronic device by touching the upper surface of cover 507 typically by a finger, depending on an indication of display element on a back surface of the touch panel. The electronic circuit detects a point of operation based on the change of a capacitance between upper conductive layer 502 and lower conductive layer 505 at a point where the user operates, and switches functions of the electronic device.
For example, while menus are displayed on the display element at the back surface, the user touches the upper surface of cover 507 typically with a finger over a desired menu. This operation causes a charge to move to the finger, and changes a capacitance between upper conductive layer 502 and lower conductive layer 505 at the point where touch panel 100 is operated. The electronic circuit detects this change of the capacitance, and selects the desired menu.
A conventional touch panel similar to touch panel 500 is disclosed in Japanese Patent Laid-pen Publication No.2009-93397.
In touch panel 500, adhesive layer 508 easily protrudes from the outer periphery of upper substrate 501 or lower substrate 504. The protruding adhesive may be attached to other part and cause stain. Or, when the upper surface of cover 507 is pressed hard typically with the finger, adhesive layer 508 at this pressed point may dent and have a pressing mark therein.